New engine init interface

[zachmu]

This lets integrators register engine initialization logic before connections are accepted.

[coffeegoddd]

@coffeegoddd DOLT

read_tests	from_latency	to_latency	percent_change
covering_index_scan	2.3	2.3	0.0
groupby_scan	144.97	144.97	0.0
index_join	1.96	1.93	-1.53
index_join_scan	1.32	1.34	1.52
index_scan	223.34	219.36	-1.78
oltp_point_select	0.25	0.25	0.0
oltp_read_only	5.0	5.0	0.0
select_random_points	0.51	0.51	0.0
select_random_ranges	0.64	0.64	0.0
table_scan	207.82	207.82	0.0
types_table_scan	467.3	467.3	0.0

write_tests	from_latency	to_latency	percent_change
oltp_delete_insert	6.21	6.21	0.0
oltp_insert	3.13	3.13	0.0
oltp_read_write	11.24	11.24	0.0
oltp_update_index	3.3	3.3	0.0
oltp_update_non_index	3.02	3.02	0.0
oltp_write_only	6.32	6.32	0.0
types_delete_insert	6.91	6.79	-1.74

[coffeegoddd]

@zachmu DOLT

comparing_percentages
100.000000 to 100.000000

version	result	total
67fd372	ok	5937471

version	total_tests
67fd372	5937471

correctness_percentage
100.0

[coffeegoddd]

@coffeegoddd DOLT

test_name	from_latency_p95	to_latency_p95	percent_change
tpcc-scale-factor-1	45.79	45.79	0.0

test_name	from_server_name	from_server_version	from_tps	to_server_name	to_server_version	to_tps	percent_change
tpcc-scale-factor-1	dolt	7bdcf6e	52.58	dolt	26ba70c	52.59	0.02

[coffeegoddd]

@coffeegoddd DOLT

comparing_percentages
100.000000 to 100.000000

version	result	total
26ba70c	ok	5937471

version	total_tests
26ba70c	5937471

correctness_percentage
100.0